The invention relates to a method to explode an encased high explosive (HE) material, and more particularly to a method for utilizing a laser to explode a metal encased HE material.
In the past there have been numerous attempts to use lasers to initiate HE materials in a variety of configurations. In order to appreciate the prior art attempts, it is necessary to provide some background information and definitions.
Types of explosions: There exist several ways in which HE material can xe2x80x9cburnxe2x80x9d that ultimately determines the magnitude of an explosion relative to the explosive potential of a given material. The most explosive event is referred to as a detonation, which indicates presence of a pressure shock wave within the HE which moves in the HE material at a speed faster than the speed of sound. In comparison, the next most explosive event is referred to as a deflagration in which the pressure shock wave associated with the burn moves in the HE material at a speed (the exact speed depending on the local pressure) which is at or below the speed of. This event may or may not be visibly distinguishable from a detonation event, depending on the burn rate of the HE material and the resulting pressure wave generated. Furthermore, a detonation typically leads to a total consumption of the HE material while a deflagration, because it is more readily quenched, may or may not consume all of the HE material. Whereas both a detonation and a deflagration can be initiated via an initial localized pressure wave, both events may also be initiated via thermal mechanisms in a very specific manner. The most common and probable thermal mechanism that leads to either detonation or a violent deflagration is a so-called slow cookoff. Slow cookoff describes a situation in which the encased HE material is heated in a slow manner causing the initial temperature to be raised in a relatively uniform fashion. Consequently, thermally induced chemical and/or structural (porosity and density) changes of the HE material take place throughout the entire volume which typically lead to a more sensitive HE material as compared to the original material. As the temperature continues to rise, gases released from the HE material increase the internal pressure of the container. Ultimately, a temperature and pressure threshold is reached near the center of the HE material that leads to an explosion (initially a deflagration that may or may not transition to detonation). In contrast, fast cookoff is a process by which the temperature of the encased HE material is raised rapidly causing only the outer perimeter of the HE material to be affected. Furthermore, the rapid rise in the temperature soon reaches an ignition (burn) temperature for the HE material causing an increase in the internal pressure. However, because the majority of the HE material has not changed in its chemistry and/or structure, it remains unchanged in terms of its ability to propagate a rapid burn. Consequently, the pressures associated with the burn are not significant enough to lead to the propagation of a self-sustained violent reaction prior to eruption of the HE casing. At the point when the casing breaks, the release of pressure typically quenches the intensity of the burn, resulting in an overall less violent explosive event.
Types of explosives: There exist a wide variety of HE materials which can be classified in several ways. Sensitive and insensitive explosives typically refer to the relative ease in which these materials may be initiated resulting in an explosive event. The specific term insensitive high explosives (IHEs) refers to a very special class of high explosives (e.g., TATB) that are very difficult to initiate, and are not the type of explosive with which the present invention is concerned. The relative sensitivity of explosives addressed in the context of warhead materials which the present invention is concerned with, only cover conventional secondary explosives and not primary explosive materials used in common igniter ordnance (e.g., lead azide). In this context, the secondary HE material is much less sensitive when compared to the detonator explosive, which requires a smaller thermal input or pressure pulse to initiate. In addition to the relative sensitivity, secondary HE materials are also categorized in terms of their formulation. Secondary HE material formulations can be described in terms of two broad categories: melt cast and pressed. Melt cast explosives are created by melting together the basic constituents of a formulation and pouring the resulting mixture into a warhead or confinement vessel. Melt cast explosives typically consist of TNT as one of the main constituents. The other broad category of secondary HE materials is so-called pressed HE material. Pressed HE materials are typically composed of a crystalline explosive material combined with a binder and compressed so large samples can be produced. In the absence of the binder, pressed HE materials will not hold their form and typically have an increased sensitivity relative to those with a binder present.
Previous accomplishments utilizing laser detonation for explosives: Prior attempts to initiate laser HE explosions can be categorized in two general forms: thermal and shock initiation. A laser initiated explosion of HE material resulting from a thermal mechanism has typically been performed on sensitive HE materials designed to respond to laser radiation. Less catastrophic events arising from thermal events on less sensitive HE material, such as fast cookoff, are the result of rapid overpressure of the confinement vessel. In this situation, almost all cases have lead to a quenched burn once the confinement vessel has ruptured (pressure vents) and the HE material has ceased to burn. In fact, this describes typical results of experiments to date attempting laser initiation of warheads. These experiments typically have used continuous wave (CW) laser operation with large field illumination simulating what has already been observed in fast cookoff testing using fuel fire as the heat source. On the other hand, direct detonation of relatively insensitive HE materials using lasers has been accomplished, but only under very specific conditions. In these cases, an extremely large pressure pulse was generated in the HE material via two general processes. The first process is to take advantage of the laser-HE material interaction in which HE material is ablated and a recoil force is generated in the HE material. In this situation, extremely large instantaneous irradiances (gigawatt/cm2 peak irradiance using nanosecond type pulses) are required to reach the minimum detonation pressures (20-200 kbar).
Although this method does not particularly require total confinement, the interaction relies on the recoil forced from ablation which varies from material to material based on the details of the HE material properties (absorption, thermal conductivity, mass densities, etc.). From a laser weapons perspective, such pulses are unlikely due to issues related to propagation in the atmosphere and laser induced material damage mechanisms. Consequently, this approach has never been pursued from the perspective of a laser weapon designed to defend against threats containing HE warheads.
The other method to achieve laser initiation of a HE material is related to laser detonators. Laser detonation of a secondary HE material typically takes advantage of confinement of the HE material and strong absorption of metallic films. A common scenario involves a sapphire window with an aluminum film on one side. The aluminum film is in contact with the HE material and the sapphire window acts as a confining element that is transparent to the laser radiation. When the laser passes through the sapphire it is absorbed by the aluminum film, which immediately generates a plasma. Because of the confinement of the sapphire window, the laser generated plasma can only propagate into the HE material, initiating a pressure pulse of sufficient magnitude to lead to a detonation wave in the HE material. In this situation, confinement is critical to achieve to pressures required for a detonation.
Prior to the present invention, attempts to detonate a metal encased HE material by impinging a laser directly on the casing have not been successful.
It is an objective of the invention to provide a method for using a laser to initiate a confined high explosive resulting in an explosion, and in particular for laser initiation of an encased high explosive.
It is a further object of the invention to provide a system utilizing a laser as a defensive weapon to initiate a warhead comprising an encased high explosive.
The above and other objects of the invention are accomplished by the provision of a method for initiating a high explosive material enclosed in a casing, comprising: generating a high energy laser beam; directing the high energy laser beam toward a location on a surface of the casing; irradiating the surface location with the laser beam for a sufficient length of time to initiate the high explosive.
Preferably, the high energy laser produces a high energy pulsed laser beam, and more preferably a pulsed laser beam having a wavelength absorbed by the casing material and a pulse width on the order 1 millisecond. By firing such a laser at a rate of 35 Hz, we have found it possible to deliver enough energy to penetrate a steel plate in a controlled environment and to initiate an explosion of a steel encased high explosive material.
It is a further aspect of the invention to provide a defensive weapon for rendering inoperable an object carrying a warhead comprised of a metal encased explosive with the use of a laser in combination with an acquisition and pointing system for aiming the laser at the object containing the warhead.